Observation of a sample surface structure is possible through detection of the secondary electron charged particles released by the sample scanned and irradiated with electrons. This is called scanning electron microscope (hereinafter, “SEM”). Another way to observe a sample surface structure is through detection of the secondary charged particles released by the sample scanned and irradiated with an ion beam. This is called scanning ion microscope (hereinafter, “SIM”).
Such surface observation preferably uses light ion species such as hydrogen and helium. Lighter ion species are preferred for their weak sputtering effect, minimizing the damage to a sample surface. Another characteristic of these ion beams is the higher sensitivity to the information of a sample surface than that of electron beams. Hydrogen and helium ions are more sensitive because the excitation region of secondary charged particles upon entry of these ions into a sample surface occurs by being localized more toward the sample surface than the excitation region occurring upon electron beam irradiation. Another disadvantage of electron beams is that the wave property of electrons cannot be ignored, and the diffraction effect causes aberration. The ion beams, on the other hand, are heavier than electrons, and the diffraction effect is negligible.
Information that reflects the inner structure of a sample can be obtained by detecting ions that passed through the sample irradiated with ion beams. This is called transmission ion microscopy. Lighter ion species such as hydrogen and helium are particularly preferred for observation because a large proportion of these ions passes through a sample upon irradiation of the sample with these ions.
On the other hand, heavy ion species such as oxygen, nitrogen, argon, krypton, xenon, gallium, and indium are preferred for the working of a sample because these ions can have a sputtering effect on the irradiated sample. A focused ion beam device using a liquid metal ion source represents a known specific example of ion beam processing devices.
A gas field ionization ion source is the preferred ion source for ion microscopy. Ina gas field ionization ion source, high voltage is applied to the metal emitter tip having an apex curvature radius of about 100 nm or less to concentrate an electric field at the apex, and a gas (ionized gas) is introduced near the apex to ionize the gas molecules in the field and obtain an ion beam. A gas field ionization ion source can generate an ion beam of a narrow energy width. Further, the small size of the ion source enables generating a fine ion beam.
Ion microscopy requires producing an ion beam of a large current density on a sample to obtain a sample image with little noise. This requires increasing the ion emission angle current density of the field ionization ion source. The ion emission angle current density can be increased by increasing the density of the ionized gas in the vicinity of the emitter tip.
Cooling the emitter tip to extreme low temperatures lowers the energy of the ionized gas molecules that collided with the emitter tip, and the ionized gas molecules aggregate and increase their density. The pressure of the ionized gas introduced near the emitter tip also can be increased. However, problems occur when the pressure of the introduced gas is 1 Pa or higher. Specifically, the ion beam neutralizes as it collides with the ionized gas, and the ion beam current decreases, or undergoes a glow discharge. A known solution to these problems is to restrict the gas ionization region with a projection of several atoms formed at the apex of the emitter tip, and improve ion emission angle current density by efficiently ionizing the limited supply of ionized gas.
Specifically, PTL 1 discloses improving ion source characteristics with a fine protrusion formed at the apex of the emitter tip.
PTL 2 discloses a charged particle microscope that enables high-resolution sample observation with a compact ion irradiation system that has a reduced ion optical length to reduce the amplitude of the relative vibrations of the emitter tip and the sample.
PTL 3 discloses an ion microscope. The main body of the ion microscope is independently installed from a cryocooler for cooling a gas field ionization ion source, and the mechanical vibration of the cryocooler that propagates to the gas field ionization ion source is reduced by the provision of a refrigerant circulation circuit cooling mechanism that circulates a refrigerant between the gas field ionization ion source and the cryocooler. In this way, the ion microscope can improve the brightness of the gas field ionization ion source while ensuring the ion beam convergence.